Method for making cathodes for electron guns

ABSTRACT

This invention relates to the manufacture of cathodes for use in electron guns. It is customary to make such cathodes with a concentrator member that forms an extension of a tube forming a housing for an electron-emissive pellet. This concentrator has an opening which diverges outwardly into the shape of a truncated cone having a predetermined geometrical position. In accordance with the invention, the housing is machined to a predetermined shape, whilst the pellet is located within said tubular housing and, as a result of this, the machined surface is co-extensive with the truncated conical opening. A plug member may be used to hold the pellet in its housing and this plug may contact the pellet through the intermediary of an annular member: this annular member may be attached to the plug. Alternatively, the plug contacts the pellet only at a limited number of points on its surface.

ilnited States Patent [191 Alais et a1.

[ METHOD FOR MAKING CATHODES FOR ELECTRON GUNS [75] Inventors: Michel Alais, Orsay; Robert Moutarde, Sevres, both of France [73] Assignee: LAir Liquide, Societe Anonyme Pour lEtude et lExploitation des Procedes Georges Calude, Paris, France [22] Filed: Apr. 28, 1972 [21] Appl. No.: 248,617

[56] References Cited UNITED STATES PATENTS 12/1961 Beck r 313/337 10/1970 Bondley 313/346 R 4/1969 Hughes 313/82 R Apr. 16, 1974 Primary Examiner-Roy Lake Assistant Examiner-James W. Davie Attorney, Agent, or Firm-Young & Thompson [57] ABSTRACT This invention relates to the manufacture of cathodes for use in electron guns. It is customary to make such cathodes with a concentrator member that forms an extension of a tube forming a housing for an electronemissive pellet. This concentrator has an opening which diverges outwardly into the shape of a truncated cone having a predetermined geometrical position. In accordance with the invention, the housing is machined to a predetermined shape, whilst the pellet is located within said tubular housing and, as a result of this, the machined surface is co-extensive with the truncated conical opening. A plug member may be used to hold the pellet in its housing and this plug may contact the pellet through the intermediary of an annular member: this annular member may be attached to the plug. Alternatively, the plug contacts the pellet only at a limited number of points on its surface.

1 Claim, 1 Drawing Figure METHOD FOR MAKING CATHODES FOR ELECTRON GUNS The present invention relates to electron guns and has for an object the provision of a method of making a cathode such that the gun shall have substantially constant perveance. Perveance is defined as the ratio between the electron current emanating from the cathode, and the voltage difference beetween the cathode, and the anode, raised to the power 3/2.

It is known that in a gun, of the Pierce type for example, the cathode is constituted substantially by a cylindrical tube arranged to support a pellet which is extremely emissive at the envisaged temperature of operation. The center of the cylindrical tube and the center of the pellet define the axis of symmetry of the gun. The tube is made from a poorly emissive material which is capable of withstanding the temperature to which the pellet is heated by any suitable means. The tube is generally extended in front of the pellet into a concentrator also made from a poorly emissive material and whose function is accurately to define the potential on both sides of the pellet and in front of the latter; maintained at the potential of the cathode, the concentrator repels the electrons emitted by the pellet, towards the longitudinal axis of the gun. Such a device has to be positioned with great accuracy with respect to the Wehnelt member, to the focussing electrodes or coils when it is desired to produce a beam having a very high power density.

It is known, moreover, that the means used for heating may be direct for example in the case of heating of the said pellet by electronic bombardment or indirect when heating of the pellet is obtained by the radiation of an interposed element. In all cases, however, heating of the pellet leads to expansion of the tube and of the pellet itself, and thus the heating causes relative movement of the latter, of small amplitude but sufficient to produce undesirable misorientation of the beam during establishment of thermal equilibrium. More particularly, relative movement of the pellet and the concentrator and the Wehnelt member frequently occurs, as a result of the difference in the coefficients of expansion of the materials.

Such a misorientation more particularly affects the perveance of the electron gun. It follows therefrom that for a constant voltage applied between the electrodes, the flow of electrons forming the beam varies in an extremely sensitive manner during the entire period of establishment of thermal equilibrium within the gun, that is to say during a period of time of the order of 20 to 30 minutes.

It has already been attempted to remedy this drawback by pre-adjusting the pellet in the cold state so that it occupies the envisaged position, when hot, at thermal equilibrium, and so that the gun thus has the calculated perveance. Besides the fact that such a method of manufacturing an electron gun can be applied only by trial and error during the construction of each cathode, it has, moreover, the major drawback of causing the perveance of thegun to vary by approximately 20 percent during the 20 to 30 minutes which follow after it has been put into operation. Moreover, it happens that electron bombardment is frequently used for welding operations of quite short duration after which it is necessary each time to stop operation of the gun.

It has also been attempted to make cathodes of pairs of materials having the following qualities: identical expansion coefficient for the two materials, great electronic emissivity at the operating temperature for the material forming the pellet, negligible electron emissivity at the operating temperature for the material forming the tube and the concentrator. Moreover, the material forming the tube has to be refractory at the operating temperature and the pellet must not break up spontaneously at this same temperature. The materials generally used to make the pellet are mixed or simple compounds such as tungsten carbide, Ranthanum lanthanium carbide or iirconium carbide, lanthanum hexaboride or a mixed carbide of tungsten, thorium and zirconium, whilst the tube supporting the pellet is formed from tungsten, tantalum or possibly rhenium. However, none of the pairs of materials forming the cathode will expand in the same manner as the concentrator or Wehnelt member even if their coefficients of expansion are equal, due to their different thermal masses and heating means applied, so that guns formed with these materials still exhibit a great variation in per- .veance during the period of establishing the thermal equilibrium, which renders them unsuitable for repeated uses where there are frequent interruptions in operation.

It is thus a more specific object of the invention substantially to remove this drawback and to produce a cathode ensuring substantially constant perveance for the gun even during the transitory phase leading to thermal equilibrium.

The invention consists in a method of making a cathode for an electron gun of the kind in which a concentrator, forming an extension of a tube having an electron-emissive pellet in a housing thereof has an opening which diverges outwardly into the shape of a truncated cone having a predetermined geometrical position, said method consisting in machining the inner surface of the housing and/or of the concentrator and inserting the pellet into the housing.

Preferably, the opening of the concentrator is machined whilst the pellet is located in the housing so that a surface of the pellet is machined and becomes coextensive with the truncated conical opening of the concentrator.

Although in cathodes produced according to known methods, the emissive pellet is forcibly inserted into its housing in the pellet-carrier tube, in the present invention the pellet is, on the contrary, freely deposited in its housing so that,even when the coefficient of expansion of the pellet differs quite substantially from the coefficient of expansion of the pellet-carrier tube, the pellet remains applied inner surface of the housing that surrounds against the the opening without being subjected to excessive pressure by the tube and its surface retains, with respect to the Wehnelt member and with respect to the electrodes, a well defined and fixed position. It follows therefrom that the electron-emitting pellet is preferably inserted without friction into its housing.

The maintenance of the position of the pellet is ensured by a plug made of the same material as the tube so as to avoid expansion problems. This plug is placed without friction in its housing and fixed to the tube by crimping or by welding. Besides its protective role, the plug plays a large part in the distribution of heat used to heat the pellet and thus avoids any inadvertent local overheating of the latter.

It may be advantageous to provide the plug with an opening enabling evacuation rearwardly of the vapours from the compound forming the emissive pellet. A certain fraction of these vapours may thus become fixed on the heating member (tungsten filament for example) which contributes to increasing its electron emissivity. It should be noted also that, in the majority of cases, it is not necessary that the plug shall rest perfectly on the rear face of the pellet and ensure perfect thermal contact because, at the operating temperature, the heat is transmitted primarily by radiation. As has already been pointed out above, the function of the plug is, principally, to hold the pellet against the truncated: conical opening. It may exert pressure on the rear face of the pellet by three small bosses provided symmetrically around the periphery of the plug, by a thin circular ring, or possibly by a washer of the same diameter as the pellet. Moreover, the plug may, according to the thickness of the pellet, project partially beyond the pellet-carrier tube or even, on the contrary, be driven into the said tube without this affecting the operation of the cathode; it follows therefrom that the thickness of the electron-emitting pellet does not need to be defined accurately which simplifies the industrial manufacture of these pellets and, as a result of this, even reduces their cost.

' In order that the invention may be more clearly understood, reference will now be made to the accompanying drawing which shows diagrammatically and in section, a cathode according to the invention.

, Referring to-the drawing, there is shown therein in section a cylindrical tube 1 which contains an electronemissive pellet 17 in a housing 5. At -2 is shown a concentrator, the nose 3 of which and the flat section 4 define the referenceplane x, y in which the front face of the emissive pellet 17 is positioned. The housing 5 has an angular clearance 6 intended to eliminate all the machining burrs and to enable the said pellet to be accurately borne on the flat circular surface 4. The inner surface of the housing also is machined. The concentrator has a conical inner machined surface 7, the angle of which is such that the sloping surface is in line with the Wehnelt membershown at 8. A plug 9 having a central opening 10 enabling evacuation rearwardly of the vapours emanating from the pellet, keeps the latter in place.

The plug 9 has on its front face three bosses, two of which are shown at 14 and 15, which are of substantially cubic shape and on which the emissive pellet 17 bears. It should be observed that this structure holds the periphery of the front face of the emissive pellet 17 strictly in place on the flat circular surface 4. The plug 9 in the example shown is welded by electron bombardment to the tube 1, the welding seam being shown at 11. The cathode is connected to the gun assembly by a conical support member 12. When it is desired to impart to the front surface of the emissive pellet a hollow, for example, a part-spherical shape, as indicated by the surface line indicated by the dotted line 13, this operation is effected by machining, taking support on the conical part 7 of the concentrator, which ensures perfect connection of the surfaces formed by the front face of the Wehnelt member 8 extended by the truncated cone 7 and terminating in a substantially part-spherical cap 13.

A device according to the above description may be made by giving the emissive surface of the pellet a diameter of the order of several centimeters but much more restricted dimensions could also be imparted thereto. This device enables a very large density of electron current to be obtained together with constant perveance at 1 percent approximately during the entire heating phase with a cathode having the following characteristics: I

a tube, made from tantalum, tungsten or rhenium, of

an initial length of 6 mm was used. This enabled a cathode to be obtained in which the useful surface of an emissive pellet made from lanthanum hexaboride has a diameter of 4 mm. The height of the emissive pellet is also 4 mm. The plug 9 has a diameter of 5.1 mm and a total height of 1 mm. The concentrator opens at an angle of the order of 50 (half angle to the apex), has a height of l mm. Such a cathode taken to a temperature of l,800l( supplies, at a voltage of 60,000 volts, a current of 0.3 amperes; the current density in the anodic hole is thus on an average approximately 13 amperes/cm". Various modifications can be made to the cathode described above without departing from the scope of the invention. The constituent used for making the emissive pellet can be replaced by others and the same applies to the materials provided for the constitution of the pellet-carrier tube. Moreover, the concentrator can be machined directly in a pellet-carrier tube as in the preceding description or be added by any known means into this tube; the plug may be formed from the same material as the tube, but it may alternatively be made from any other refractory material, more particularly from a porous material. Finally, these two members can be replaced by others which would act in the same technical manner without departing from the scope of the invention.

What we claim is:

1. In a method for making a cathode for an electron gun, which cathode comprises a tube, a concentrator in the form of an annular radially inwardly extending flange on the forward end of the tube, said flange having an annular forwardly opening concave surface thereon, an electron-emissive pellet in the tube, said pellet bearing against said flange, and a plug secured in the tube and bearing against only a portion of the rear of said pellet to retain said pellet in said tube; the improvement comprising machining the surface of said flange against which said pellet bears, inserting said pellet freely in said tube without friction, and maching the forward surface of said pellet to the contour of said concave surface so that the forward surface of said pellet is coextensive with said concave surface.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORECTION Patent No. 3,803,677 Dated pril 16, 1974 fig Michel Alais et al .'-Inventor(s) i It is certified that error appears in the above-identified patent and that said. Letters Patent are hereby corrected as shown below:

Column 2, rewrite line 53 as follows remains applied against the inner surface of the housing that SUI" Signed and sealed this 18th day of February 1975.

(SEAL) Attest:

C. MARSHALL DANN RUTH C. MASON Commissioner of Patents Attesting Officer and Trademarks FORM PC4050 ($69) uscomwoc 60376-P69 U.S. GOVERNMENT PRINTING OFFICE 869 93 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTIDN April 16, 1974 Patent No. 3,803,677 Dated 'Invmnmr(S) Michel Alais et a1.

It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

On the cover sheet item [73} "Calude" should read claude Signed and sealed this 8th day of October 1974.

(SEAL) Attest:

McCOY M. GIBSON JR. 0. MARSHALL DANN Attesting Officer Commissioner of Patents USCOMM-DC 60376-P69 u s covcwnem PRINTING orncz; 869 93 o 

1. In a method for making a cathode for an electron gun, which cathode comprises a tube, a concentrator in the form of an annular radially inwardly extending flange on the forward end of the tube, said flange having an annular forwardly opening concave surface thereon, an electron-emissive pellet in the tube, said pellet bearing against said flange, and a plug secured in the tube and bearing against only a portion of the rear of said pellet to retain said pellet in said tube; the improvement comprising machining the surface of said flange against which said pellet bears, inserting said pellet freely in said tube without friction, and maching the forward surface of said pellet to the contour of said concave surface so that the forward surface of said pellet is coextensive with said concave surface. 